PETIT-SPOT AND GEOLOGY IN NW PACIFIC

Submarine petit-spot volcanoes occur on region of the plate-flexure prior to subduction (Hirano et al., 2006; 2013). The distribution of petit-spot monogenetic volcanoes are influenced by the tectonic stress field in the lithosphere. Such tiny volcanoes are ubiquitous on the portion of plate flexure in the world, which are recently reported from oceanward slope of Tonga, Chile and Java Trenches (Hirano et al., 2008; 2013; Taneja et al., 2014). They are not only the portion of outer-rise warping on lithosphere prior to subduction, which is extensional Basin and Range (Valentine & Hirano, 2010) and south of Greenland (Uenzelmann-Neben et al., 2012). The spatial distributions of petit-spot are still enigmatic. Although submarine tiny volcanoes should appear homogeneously on the submarine surface wherever the plate is flexing, the area of absolutely no volcanoes and lavas have found from the surrounding areas of already reported petit-spots on Sites A, B and C off Tohoku, Japan (Hirano et al., 2006). The additional local reasons in lithosphere could be necessary to occur the petit-spot volcanoes. Here we report the distributions of petit-spot controlled by the tectonic fabrics of seafloor, and the nine submersible dives at the Site C conducted by JAMSTEC submersible Shinkai6500 on this April.

Three principal tectonic fabrics appear on subducting Pacific Plate off the Japan Trench, which are 1) ridge-perpendicular fabrics, 2) ridge-parallel abyssal hills, and 3) horst and graben structures. At the trench oceanward slope, fabric#1 is approximately parallel to neighbor Nosappu and Kashima fracture zones (Nakanishi, 1993). The trends of Japan Trench axis are changed from N-S in the north to NE-SW in the south here, where the structures of fabric#3 are complicatedly intersecting (Nakanishi et al., 2011). The Shinkai6500 observed the 10 volcanoes in more than 80 petit-spots in Site C (Hirano et al., 2008). Alterations of quenched glass of gained samples show the petit-spot eruptions at the several times during 0 to 10 Ma. As fabric#1 and #2 along petit-spot distribution are clearly original structure before subduction-related outer-rise, the petit-spot eruptions are probably related to re-activation of traditional structures due to the subduction-related bending of NW Pacific Plate.